The present invention relates to devices that utilize ultrasound to determine the direction and speed of a fluid flowing in a vessel, and more particularly to Doppler diagnostic medical systems and methods for measuring blood flow.
Doppler ultrasound measurements of flow, widely used for blood flow measurement in medical applications, and for the measurement of other scattering fluids in industrial applications, depend upon the Doppler effect, whereby a scatterer produces a change in the frequency of the ultrasound that it scatters. This change in frequency is proportional to two unknown quantities: the absolute magnitude of the velocity vector characterizing the motion of the scatterer, and the angle between the velocity vector and the insonating beam.
By simultaneously making two Doppler measurements of a velocity whose vector is coplanar with the transducer using two beams at known angles to each other, the resulting Doppler equations (each of which contains the unknown quantities of absolute value V and angle in the plane xcex8) can be solved simultaneously to calculate the velocity and angle to the transducer of that vector.
Determining three vector components of velocity by means of multiple Doppler equations has also been discussed, for example, U.S. Pat. No. 5,738,097 issued to Beach et al, and as discussed in the referenced patents U.S. Pat. No. 5,488,953 and U.S. Pat. No. 5,540,230. These patents taught apparatus and methods useful for pulsed Doppler, rather than CW Doppler. For certain applications where skilled operators to interpret the image in order to place the sampling gate needed for pulsed Doppler are not available (as for primary care screening for disease), CW Doppler is desirable. U.S. Pat. No. 4,062,237, issued to Fox, utilizes crossed CW beams and multiple frequencies where pairs of transducers operate at different frequencies so as to set up a difference frequency standing wave in the region of interest (equivalent to sensitive volume in this disclosure) in order to detect a Doppler frequency.
The method of using multiple Doppler measurements to determine the vector components of the velocity has been used by Daigle (1974 Doctoral Dissertation, Colorado State University) (unpublished) and implemented in previous patents, such as U.S. Pat. No. 5,488,953 xe2x80x9cDoppler Diffracting Transducerxe2x80x9d and U.S. Pat. No. 5,540,230 entitled xe2x80x9cDoppler Diffracting Transducerxe2x80x9d, both issued to Vilkomerson, the inventor herein. These patents, in addition to issued U.S. Pat. No. 6,346,081 (""081) entitled xe2x80x9cAngle Independent Continuous Wave Doppler Devicexe2x80x9d have disclosed means and methods of using special transducers, known as diffraction-grating-transducers (DGTs), to generate the multiple beams needed to effect this method. U.S. Pat. Nos. 5,488,953 and 5,540,230 teach the use of these transducers for pulsed operation, and patent ""081, incorporated herein by reference, describes using these transducers for continuous wave (CW) operation. CW operation is often desirable for medical and some industrial uses because CW operation does not require adjustment of a xe2x80x9csample gatexe2x80x9d to define the spatial region in which the Doppler system will measure the velocity. Instead, the region where the beams overlap define the xe2x80x9csensitive regionxe2x80x9d. Patent ""081 describes how this sensitive region is determined for CW operation.
In application U.S. Ser. No. 10/164,446, by Vilkomerson, the inventor herein, which is incorporated herein by reference, a CW, angle-independent system that is orientation independent is taught. As shown in FIG. 1, taken from the referenced application, it utilizes three (or more) Doppler measurements arranged so that the measurements involve the three spatial components of velocity, i.e. Vx, Vy and Vz. Once the three components are determined, the absolute velocity V can be calculated as equal to (Vx2+Vy2+Vz2).
Using such a transducer is particularly desirable when measuring blood flow under the skin, where the orientation of the blood vessel is not obvious. With the transducer such as that shown in FIG. 1, the velocity will be accurately determined independently of the orientation, as was demonstrated experimentally in xe2x80x9cLow-Cost Vector Doppler System Utilizing Diffraction-Grating Transducersxe2x80x9d, by Vilkomerson et al., Proc. 2000 IEEE International Ultrasonics Symposium, IEEE Press, Piscataway (2001). Details of an instrument are provided there by which the three independent Doppler frequency signals containing three corresponding unknown velocity components in three spatial dimensions, i.e. Vx, Vy, and Vz, are analyzed in order to obtain the velocity in terms of the three measured Doppler frequencies.
Shortcoming of present vector Doppler methodsxe2x80x94This method works well when a single velocity vector is present, such as when an artery or a vein is examined. However, there is an important medical application of Doppler use where more than one blood vessel is present: examining the carotid bifurcation for the presence of significant stenosis there. It has been shown that if plaque at this area reduces the diameter of the carotid artery to 40% or less (a xe2x80x9c60%+stenosisxe2x80x9d), the risk of a serious stroke over 5 years rises to over 10%, and removing that plaque by means of an operation, a carotid endarterectomy, reduces the risk by over 50% (xe2x80x9cEndarterectomy for asymptomatic carotid stenosisxe2x80x9d. Journal of AMA 273:1421-8, 1995). When such a stenosis is present, the area of the carotid is reduced to 0.16 of its normal area, and so the velocity increases inversely proportionally to the area; this increase of velocity can be easily detected by the Doppler signal that arises from the stenosis. While Doppler has been shown to be very effective at finding these stenoses (see, for example, Huston J., et al, xe2x80x9cRedefined duplex ultrasonographic criteria for diagnosis of carotid artery stenosisxe2x80x9d, Mayo Clin Proc 75:1133-1140 2000) screening for this condition has not taken place because of the expense and operator-dependence of conventional duplex Doppler studies of the carotid. If a simple Vector Doppler transcutaneous examination were practical, such screening could take place.
However, the method of Vector Doppler, as described by Daigle, and others does not give accurate answers when applied to the carotid bifurcation. To understand why, we take a simple example: the xe2x80x9ccrossed-beamxe2x80x9d system shown in FIG. 2. In this configuration for Vector Doppler (see patent by Beach, et al., already referenced) a pair of transducers is on the x-axis and a pair on the y-axis. If moving blood, represented by a velocity vector, exists in the shaded area shown in the Figure, its components, Vx and Vy can be found from the Doppler frequencies as shown in FIG. 2. When the maximum component Vx and Vy is found (by simply subtracting the frequencies as shown), the square root of the sum of the squares will provide the maximum velocity.
When two velocity vectors are present in the sensitive region, as would be expected where the carotid bifurcates, the method described above fails. Assume two equal velocity vectors, of value 100 cm/sec being present, one velocity vector along the x-axis, and a second one along the y-axis; the x-axis pair of transducers will provide the proper velocity for the x-vector, and the y-axis pair for the y-vector. When the velocities are squared, summed, and square-rooted, however, the answer will be 141 cm/sec, i.e. the 2 times the true maximum velocity. The basic assumption of the conventional vector Doppler method, that the velocity can be found by finding the spatial components and vector summing, does not hold for multiple velocity vectors occurring simultaneously. These conventional vector Doppler methods cannot be used for the important medical need of carotid screening, given the anatomy of the carotid bifurcation and this shortcoming.
We describe here a new configuration of vector ultrasound Doppler with CW beams that has the important advantage of detecting the highest velocity in its xe2x80x9csensitive volumexe2x80x9d, even when multiple velocities are present in the sensitive volumexe2x80x94a situation that causes the vector Doppler systems previously disclosed to give incorrect results. The new configuration is particularly well suited for screening, i.e. searching for high velocities indicative of stenoses, in the carotid bifurcation region (where the anatomy of the bifurcation ensures the existence of multiple velocity vectors).
The new invention can utilize diffraction-grating transducers for the advantages described in pending application U.S. Ser. No. 10/116,446: their capability of changing the angle of the beam produced by a change in the driving frequency, or by changing the phase relation among the sub-grids of the diffraction-grating transducer, as described in the referenced patents. The new invention can also use physically angled transducers emitting beams at perpendicular angle to its face, as shown in FIG. 3. The descriptions that follow will show the physically-angled conventional transducers for ease of discussion; however, the diffraction-grating transducer can be understood to provide a superior alternative to the conventional transducer in the discussion of the invention that follows.